Conventional photovoltaic systems, in which sunlight is directly converted into electricity by solar cells, suffer from two major drawbacks. First of all, they generally stop working after sunset when the electricity they are designed to generate is needed most. This intermittent nature also makes it difficult to integrate solar energy sources into existing power grids. Second, solar cells usually can only absorb light within a narrow band of wavelengths (e.g., in the near infrared (IR) band) out of the broadband spectrum of solar radiation, potentially limiting the overall conversion efficiency from optical energy to electrical energy.